A head-mounted display, both abbreviated HMD, is a display device, worn on the head or as part of a helmet, that has a small display optic in front of one or each eye. A HMD has many uses including in gaming, aviation, engineering, there is also an optical head-mounted display, which is a wearable display that can reflect projected images and allows a user to see through it. A typical HMD has one or two small displays, with lenses and semi-transparent mirrors embedded in eyeglasses, a visor, or a helmet, the display units are miniaturised and may include cathode ray tubes, liquid crystal displays, liquid crystal on silicon, or organic light-emitting diodes. Some vendors employ multiple micro-displays to increase resolution and field of view. HMDs differ in whether they can display only computer-generated imagery, or only live imagery from the physical world, most HMDs can display only a computer-generated image, sometimes referred to as a virtual image. Some HMDs can allow a CGI to be superimposed on a real-world view and this is sometimes referred to as augmented reality or mixed reality. Combining real-world view with CGI can be done by projecting the CGI through a partially reflective mirror and this method is often called optical see-through. Combining real-world view with CGI can also be done electronically by accepting video from a camera and this method is often called video see-through. An optical head-mounted display uses an optical mixer which is made of partly silvered mirrors and it can reflect artificial images, and let real images cross the lens, and let a user look through it. Various methods have existed for see-through HMDs, most of which can be summarized into two families based on curved mirrors or waveguides. Curved mirrors have been used by Laster Technologies, and by Vuzix in their Star 1200 product, various waveguide methods have existed for years. These include diffraction optics, holographic optics, polarized optics, major HMD applications include military, government, and civilian-commercial. In 1962, Hughes Aircraft Company revealed the Electrocular, a compact CRT, ruggedized HMDs are increasingly being integrated into the cockpits of modern helicopters and fighter aircraft. These are usually integrated with the pilots flying helmet and may include protective visors, night vision devices. Military, police, and firefighters use HMDs to display information such as maps or thermal imaging data while viewing a real scene. Recent applications have included the use of HMD for paratroopers, in 2005, the Liteye HMD was introduced for ground combat troops as a rugged, waterproof lightweight display that clips into a standard US PVS-14 military helmet mount. The self-contained color monocular organic light-emitting diode display replaces the NVG tube, the LE has see-through ability and can be used as a standard HMD or for augmented reality applications. The design is optimized to provide high definition data under all lighting conditions, the LE has a low power consumption, operating on four AA batteries for 35 hours or receiving power via standard Universal Serial Bus connection
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